The invention relates to the use of certain carboxylic acid esters and polyvalent alcohols as extracting agents for obtaining pure polyesters or copolyesters containing 3-hydroxybutyric acid units.
Poly-D(-)-3-hydroxybutyric acid (poly-HB) is synthesized and accumulated inside the cell by many microorganisms as a substance for storing energy and carbon and represents a polyester having thermoplastic properties which is biologically degradable. Poly-HB can, for example, be prepared in good yields without problems by the procedures described in U.S. Pat. No. 4,786.598.
Copolyesters of poly-HB, such as, for example, copolyesters consisting of 3-hydroxy-butyric acid and 3-hydroxyvaleric acid units and also other acid units should, according to EP-A-0,052,459, exhibit better processing properties than pure poly-HB when used as thermoplastics. A process for the preparation of such copolyesters is disclosed in EP-A-0,069,497.
The polyesters are present in the cell material of the microorganism after biological preparation and must then be extracted from the cell material. Until now, this caused considerable difficulties.
Thus, for example, in U.S. Pat. No. 3,036,959 or in U.S. Pat. No. 3,044,942, it is described that good yields in the extraction of the polyester from the cell material of the microorganism can only be achieved if an additional step for disrupting the cells, in which the cells are treated with acetone, is introduced before the actual extraction step. Pyridine or methylene chloride must be used as extracting agents.
In U.S. Pat. No. 3,275,610, chloroform is described as an extracting agent. In order to achieve good yields, however, the cells must be treated for a very long time with the extracting agent. However, due to the long treatment, depolymerization of the poly-HB occurs so that either a poor yield or a reduction in the molecular weight of poly-HB must be taken into account in this method.
In U.S. Pat. No. 4,310,684, other halogenated hydrocarbons are proposed for the extraction. However, halogenated hydrocarbons are on the whole toxic and represent a hazard for any who have to work with them, and additionally a pollution of the environment, it also having to be taken into consideration that residual contents of this solvent in the isolated poly-HB are unavoidable.
In U.S. Pat. No. 4,101,533, cyclic carbonic acid esters such as ethylene carbonate or propylene carbonate were therefore proposed as solvents for polyhydroxybutyric acid. However, these solvents are very corrosive in the hot state in which they have to be used and attack taps and joints of apparatuses. A relatively long treatment of the cells with ethylene carbonate or propylene carbonate is necessary for a good yield in the extraction of poly-HB, a particularly large reduction in the molecular weight of the poly-HB or its copolyesters occurring, however, which has disadvantageous consequences for the use of poly-HB or its copolyesters as thermoplastics.
In contrast, solvents for the simple and problemfree extraction of poly-HB and its copolyesters could now be found, the use of which as extracting agents avoids the abovementioned disadvantages, the polyesters or copolyesters being obtained in an unexpected high purity of at least 98% in very good yields.